Wrist pin for piston and connecting rod assembly

ABSTRACT

Metallic piston, wrist pin and connecting rod which normally do not fit together can be joined with one selected component being heated to effect the expansion thereof while another selected component is chilled to effect its contraction. With the heat differentials effecting relative size change the components can be loosely put together in a preassembly. The components in the loose preassembly are then returned to ambient temperature and to specification size whereby a central journal of the wrist pin is trapped between the piston pin bosses in one embodiment and the connecting rod is trapped by the shoulders of a reduced diameter pin journal in a second embodiment to form permanent piston, connecting rod and wrist pin assemblies.

TECHNICAL FIELD

This invention relates to piston wrist pin and connecting rod assembliesfor stroking operation in associated cylinders, and more particularly,to new and improved wrist pin connections for the articulation ofpistons to associated connecting rods and to new and improved heatdifferential methods for connecting pistons to connecting rods.

BACKGROUND OF THE INVENTION

In many piston powered units, such as compressors and internalcombustion engines, full floating piston or wrist pins are used tooperatively connect connecting rods with pistons for operating in anassociated cylinder. Such pins advantageously provide two stages orlevels of pin freedom for reducing pin and bearing friction and wear.When properly installed in a piston and rod assembly, these pins possessthe capability of turning in the pin bearing, or bores, of laterallyspaced bosses of the piston and also relative to the pin bearing orbushing at the small end of the connecting rod through which the pinextends.

Axial thrust loads applied to wrist pins, such as from the sidewaysmovement of the connecting rod during piston stroking in the associatedcylinder, may cause the wrist pin to slide axially in the pin bores. Ifthe pin is displaced to a point at which one end of the pin physicallycontacts and rubs against the cylinder wall, the cylinder and the pistonare subject to damage often necessitating labor intensive and expensiverepair.

To prevent such occurrences, wire end locks or snap rings areoperatively mounted in the bores of the pin bosses at outboard of theends of the pin to trap the pin in a substantially centered position.With the pin so held within confines of the pin boss bores and away fromthe cylinder walls, pin damage to the cylinder and piston is obviated.An example of such pin retention is found in Auto Service and Repair, M.W. Stockel et al., The Goodheart-Willcox Co., Inc., Page 280, C 1978.

In addition to wire end locks, a wide variety of other retainers havebeen used for similar purposes. For example, retainer clips such asshown in U.S. Pat. No. 5,367,945, issued Nov. 29, 1994 to Halka, et al.,and split and two piece bearings, such as shown in U.S. Pat. No.1,491,151, issued Apr. 22, 1924 to L. J. McKone, are used to securewrist pins in operative position.

While these pin retention structures and methods may be satisfactory forsome installations, they require special bearing structures or separateretention components which add to the cost burden of piston andconnecting rod assemblies and are subject to failure, misassembly, andshortened service life, particularly, since they generally operate in aharsh environment such as in internal combustion engines.

SUMMARY OF THE INVENTION

The wrist pin, piston and connecting rod construction and method of thisinvention eliminates engine failures caused by broken or misassembledpin retainers, such as lock rings, by improving the connection of thewrist pin in operating position so that the retainers or lock ringswould be redundant and accordingly not used.

In one embodiment, an intermediate journal of a generally cylindricalwrist or piston pin is ground with a 0.095 mm stepped diameter (outerdiameter minus inner diameter) profile on its outer diameter. The pistonpin boss bores are machined to a diametrical dimension approximately0.081 mm less than the large OD of the intermediate journal pin. Inorder to assemble the components, the pin is chilled in liquid nitrogen(-132° C.) which shrinks the pin OD. At this time, the piston is heatedto 176° C. which expands internal diameter of the pin boss bores. Thepin having a reduced diameter is easily inserted through the expandedbores of the pin bosses, and through the bushing of the connecting rodaligned between the pin bosses until the intermediate pin journalcenters in the bushing. The connecting rod bushing does not require anyspecial machining or assembly process in this embodiment. The componentsof the assembly are subsequently returned to ambient temperature, thepin will return to its designed dimension and be retained in the pistonby the stepped intermediate journal since it is now larger than theinternal diameters of the bores of the pin bosses. This stepped journalwill also retain the pin in a running engine since the piston and pintypically operate within 10° C. of each other.

The pin will be free to rotate in both the piston pin boss bores and theconnecting rod bushing as in the traditional full floating design. Withthe wire lock rings and other fasteners eliminated and with the wristpin centrally trapped between the pin bosses in a positive manner,engine failures related to improper installation and failure of lockrings as well as other retainers are eliminated.

Another preferred embodiment with the same advantages is similar to thatdescribed except that the center or intermediate journal of the wristpin is reduced in diameter. The assembly of the pin, rod and piston isalso similar. The pin is again chilled in liquid nitrogen, but insteadof heating the piston, the small end of the connecting rod is heated.This effects the relative contraction and expansion of these componentsso that they fit together for easy assembly. After the pin is insertedthrough the pin boss bores of the piston and the bushing of theconnecting rod, the components are brought back to ambient temperatureas previously described. In this design, the reduced diameterintermediate journal closely fits for turning within the bushing of thesmall end of the rod, and the end journals at opposite end of the pinhaving large diameters traps the connecting rod therebetween. Thelaterally spaced inner faces of the pin bosses trap the connecting rodand the wrist pin in a centralized position. Once assembled, thecentrally trapped pin is free to rotate in both the piston and theconnecting rod, but has limited axial movement so no lock rings or otherpin retainers are required. The difference between this and the firstdescribed design is the location where axial thrust is applied. In thispreferred design the pin, if axially loaded, would rub up against theconnecting rod bushing which may be an alloy of steel and bronze,whereas in the previous design, the large diameter intermediate journalpin would rub against the pin bosses of the piston usually of analuminum alloy.

The pin designs of this invention eliminates costs associated withauxiliary retainers. For example, the invention eliminates the cost ofthe lock rings; the machining of the lock ring grooves in the pistonbosses; the grinding of tight tolerance chamfers on the outer ends ofthe pin; and the cost of installing the lock rings. Furthermore, thepotential for broken and improperly installed pin retainers iseliminated and damage to the piston and cylinder walls from loose pinsis eliminated or substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a piston, connecting rod and wrist pinassembly for an internal combustion engine;

FIG. 2 is a diagram illustrating a first preferred assembly of a piston,connecting rod and wrist pin according to this invention;

FIG. 2a is a pictorial view of the wrist pin of FIG. 2;

FIG. 3 is a sectional view of a piston, connecting rod and wristassembly according to the assembly method of FIG. 2;

FIG. 4 is a diagram illustrating a second preferred method of assemblyof a piston, connecting rod and wrist pin;

FIG. 4a is a pictorial view of the wrist pin of FIG. 4, and

FIG. 5 is a sectional view of a piston, connecting rod and wrist pinassembled in accordance with the assembly method of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now in greater detail to the drawings, there shown in FIG. 1, apiston and connecting rod assembly 10 for an internal combustion enginecomprising a generally cylindrical piston 12, preferably of any aluminumbased alloy articulated to an elongated connecting rod 14 by a generallycylindrical wrist pin 18 both of which are preferably steel. The piston12 has a pair of downwardly extending and laterally spaced bosses 20, 22which form inner structure thereof. The bosses have inner shoulders 24,26 which are separated from one another to provide the space 27 toaccommodate the pivotal upper end 28 of the connecting rod 14. Thebosses 20, 22 have aligned bores 30 and 32 formed therein which serve asbearings for the wrist pin 18.

As shown more particularly in FIGS. 2a and 3, the wrist pin 18 has largediameter outboard end journals 36, 38, at opposite ends of a smalldiameter intermediate journal 40, which respectively rotatably mount inpin boss bores 30, 32 while the small diameter steel and bronze bushing42 pressed into the upper end 28 of connecting rod 14 provides forrelative turning movement between the pin and connecting rod 14.

The lower end of the connecting rod is formed with semi-cylindrical yoke46 and has a semi-cylindrical cap 48 secured thereto by threadedfasteners 50 to define the "big end bore" 52 which connects to a crankpin of the crankshaft of an engine, not shown.

The wrist pin preferably has two degrees of freedom, that is the pin canrotate within the bearings provided by the bores of the pin bosses, andfurthermore, can turn relative to the bushing of connecting rod 14. Withsuch construction, the frictional wear of the pin while the piston isoperating in its cylinder is reduced to provide for an improvedconnection with longer service life.

The present invention provides an improved method of assembly. Normally,the steel-bronze bushing 42 of connecting rod 14 has an inner diameter54 which is just slightly greater than the outer diameter 56 of therecessed intermediate journal 40 of pin 18 to provide for articulation.

Because of the larger diameters of the two equally sized end journals36, 38 of the pin 18, it would be normally difficult to pass the pin 18through the smaller diameter bushing 42 of the connecting rod 14. Moreparticularly, at ambient temperatures, there would be such interferencebetween the outer journals 36, 38 of the pin 18 and the internaldiameter 54 of the rod bushing 42 that axial insertion of the outer pinjournals through the bushing at ambient temperature would not bepossible.

However, to facilitate such an assembly, the pin 18 is moved from partsstation A, to the left when viewing in FIG. 2 and placed into a chillingunit 64 at treatment position B which in the preferred embodimentcontains liquid nitrogen.

There the pin 18 is chilled to effect the contraction and shrinkagethereof. In particular, the diameters of the outer pin journals are 36,38 reduced so that they can be easily inserted through the journalsprovided by the bores of the piston pin bosses 20, 22 and the free endthereof through the bushing 42 of the rod 14 which has been expanded ashereinafter described.

More particularly, this pin insertion and installation is augmentedbecause the rod 14 and its bushing 42 have been heated and expanded inthe oven 66 at treatment station B, as shown in FIG. 2, to expand theinternal diameter of the bushing. After such treatments, the connectingrod 14, piston 12 are moved to assembly station C, where the pin boresof the bosses are axially aligned with the bushing 42 of the rod 14.With this alignment and heat and chill treatments, the pin 18 can besuccessfully inserted axially to the centralized position shown in FIG.2. In this position the bushing 42 of the connector rod 14 isintermediate the end journals 36, 38 and is disposed around the smalldiameter intermediate journal 40 of the pin.

To provide for trapping of the connecting rod onto the pin and the pinand rod in the piston, the pin and rod are then brought to an ambienttemperature, i.e., 70° F. so that the pin 18 expands to specificationand the connecting rod contracts to specification to trap the pin to theconnecting rod. As best illustrated in FIGS. 2a and 3, inwardly facingannular shoulders 70 and 72 of the end journals of the pin at ambientand working temperatures are dimensioned to contact the end bushing 42and the surrounding area of the rod 14, if desired, and block and limitsideways movement of the rod 14 relative to the pin. While theconnecting rod 14 is trapped on the pin 18, the rod and the pin are alsotrapped between the two faces or shoulders 24, 26 of the depending pinbosses 20, 22 so that the assembly is completed and is operativelymounted together.

In this arrangement, the heating of the rod and the contraction of thepin does not adversely change the grain structure of the metals of theseelements. Furthermore, when operating in an engine, the expansion of theparts from the heat of combustion does not significantly change therelative diameters of these bearings, bushing and journals so that thepin retains its assembled configuration, as shown in FIG. 3.

The following is one illustration of the treatment and installation ofthe steel pin 18 into the bores of the aluminum based piston bosses andthe steel-bronze connecting rod bushing:

    ______________________________________                                        AMBIENT DIMENSIONS OF COMPONENTS                                              ______________________________________                                        PIN OD @ OUTER END JOURNAL                                                                           24.040 mm                                              PIN OD @ CENTER JOURNAL                                                                              23.997 mm                                              ROD BUSHING ID         24.008 mm                                              PISTON PIN BOSS BORE ID                                                                              24.054 mm                                              AMBIENT PIN TO PISTON BOSS                                                                           0.014 mm                                               CLEARANCE                                                                     AMBIENT PIN TO ROD BUSHING                                                                           0.011 mm                                               CLEARANCE                                                                     ______________________________________                                        INSTALLING THE PIN                                                            PIN JOURNAL                                                                   AT AMBIENT   CHILLED   RADIAL    CHILLED                                      TEMPERATURE  PIN       CHANGE    DIMENSION                                    (outer end) OD (mm)                                                                        TEMP °C.                                                                         EXP. (mm) OD (mm)                                      ______________________________________                                        24.040       -132      -0.020    24.020                                       ROD BUSHING AT                                                                             (-270° F.)                                                ______________________________________                                        AMBIENT      HEAT      RADIAL    HEATED                                       TEMPERATURE  ROD       CHANGE    DIMENSION                                    ID (mm)      TEMP °C.                                                                         EXP. (mm) ID (mm)                                      ______________________________________                                        24.008       232       0.038     24.044                                                    (450° F.)                                                 ______________________________________                                        INSTALLATION CLEARANCE:                                                       MAX. DIAMETER PIN TO ROD BUSHING                                              ______________________________________                                        0.024 mm                                                                      ______________________________________                                        IN A RUNNING ENGINE                                                           AMB. PIN END                                                                             RUNNING PIN  RADIAL    RUNNING                                     OD (mm)    TEMP °C.                                                                            EXP. (mm) OD (mm)                                     ______________________________________                                        24.040     180          0.028     24.068                                                 (358° F.)                                                   ______________________________________                                        AMB. BUSHING                                                                             HEAT PISTON  RADIAL    HEATED                                      ID (mm)    TEMP. °C.                                                                           EXP. (mm) ID (mm)                                     ______________________________________                                        24.008     190          0.029     24.037                                                 (374° F.)                                                   ______________________________________                                        SHOULDER STEP RETAINING THE PIN                                                                         0.031 mm                                            IN THE PISTON DURING ACTUAL ENGINE                                            RUNNING                                                                       ______________________________________                                    

Referring now to the embodiment and method of FIGS. 4 and 5, the piston80 and rod 82 may be same as in the first embodiment. However, in thisarrangement, the steel pin 84 is formed with a large diameterintermediate journal 86 and smaller diameter end journals 88, 90,preferably of equal diameters. The internal diameter of the bores 92, 94of the laterally spaced bosses 96, 98 of the piston 80 are increased intheir internal diameters from ambient specification diameters by movingthe piston 80 into an oven 100 at station B and adding heat energythereto to effect piston expansion. As the internal diameter of thebores of the piston bosses are being increased, the pin 84 is beingchilled in chilling unit 102 and reduced in diameter.

The piston 80 and piston rod 82 are placed in a suitable fixture, notshown at assembly station C. The chilled pin 84 is inserted axially intothe bores 92, 94 of the pin bosses and through the aligned bushing 106of the rod. The pin is thus in a centralized position relative to therod and piston, as shown in FIG. 4.

The assembled parts are then allowed to return to ambient temperature sothat the piston bosses and their bores contract while the pin expands.Under such conditions, the pin is trapped between the bores of the pinbosses with the intermediate journal 86 being of large diameter andproviding shoulders 108, 110 on either side thereof. Those shoulderscontact the shoulders 112, 114 of the pin bores to trap the pin in acentralized position. Since the connecting rod has been insertedthereon, the connecting rod is trapped in the piston for turningoperation thereon on the pin.

The following chart illustrates the treatment and installation of thepin 84 into the piston boss bores and connecting rod bushing to completethe assembly, illustrated in FIG. 5:

    ______________________________________                                        AMBIENT DIMENSIONS OF COMPONENTS                                              ______________________________________                                        PIN OD @ OUTER JOURNALS  23.905 mm                                            PIN OD @ CENTER JOURNAL  24.000 mm                                            ROD BUSHING ID           24.011 mm                                            PISTON PIN BOSS ID       23.919 mm                                            AMBIENT PIN TO PISTON BOSS BORES                                                                       0.014 mm                                             CLEARANCE                                                                     AMBIENT PIN TO ROD BUSHING                                                                             0.011 mm                                             CLEARANCE                                                                     ______________________________________                                        INSTALLING THE PIN                                                            AMB. PIN CENTER                                                                             CHILL                                                           JOURNAL       PIN       RADIAL    CHILLED                                     OD (mm)       TEMP °C.                                                                         EXP. (mm) OD (mm)                                     ______________________________________                                        24.000        -132      -0.017    23.966                                                    (-270° F.)                                               ______________________________________                                                       HEAT                                                           AMB. PIN BOSS BORE                                                                           PISTON    RADIAL    HEATED                                     ID (mm)        TEMP °C.                                                                         EXP. (mm) ID (mm)                                    ______________________________________                                        23.919         176       0.040     23.998                                                    (350° F.)                                               ______________________________________                                        INSTALLATION CLEARANCE:                                                       MAX PIN DIAMETER TO PIN BOSS BORE                                             ______________________________________                                        0.032 mm                                                                      ______________________________________                                        IN A RUNNING ENGINE                                                           PIN CENTER AT                                                                 AMBIENT     RUNNING    RADIAL    RUNNING                                      TEMPERATURE PIN        CHANGE    DIMENSION                                    OD (mm)     TEMP °C.                                                                          EXP. (mm) OD (mm)                                      ______________________________________                                        24.000      180        0.024     24.049                                       (358° F.)                                                              ______________________________________                                        PIN BOSS BORE AT                                                              AMBIENT      HEAT      RADIAL    HEATED                                       TEMPERATURE  PISTON    CHANGE    DIMENSION                                    ID (mm)      TEMP. °C.                                                                        EXP. (mm) ID (mm)                                      ______________________________________                                        23.919       190       0.043     24.006                                                    (374° F.)                                                 ______________________________________                                        SHOULDER STEP RETAINING THE PIN IN                                                                      0.043 mm                                            THE PISTON DURING ACTUAL ENGINE                                               RUNNING                                                                       ______________________________________                                    

While the pin is retained in the piston, should repairs or replacementbecome necessary, the pin can be conventionally removed and repairseffected.

Accordingly with this invention, no supplemental ring, threadedfasteners, clips, or other fasteners are needed and the pin is retainedwith optimum assurance in operating position with minimal number ofparts.

While a preferred embodiment of the invention has been shown anddescribed, other embodiments will now become apparent to those skilledin the art. Accordingly, this invention is not to be limited to thatwhich is shown and described but by the following claims.

I claim:
 1. A piston, connecting rod and wrist pin assembly comprising agenerally cylindrical metallic piston member, an elongated metallicconnecting rod member, a unitary wrist pin of metal having axiallyspaced cylindrical end journals of a first outer diameter integraltherewith and formed at opposite outer ends thereof and further having acylindrical connecting rod journal of a second outer diameter formedbetween said end journals, said connecting journal being operativelydifferent in outer diameter as compared to the outer diameter of saidend journals, said piston member having a pair of laterally spaced pinbosses, each of said pin bosses having a circular opening of apredetermined diameter therein to provide a pair of axially spacedbearings of equal internal diameters which are axially aligned with oneanother, said spaced bearings of said pin bosses being sized torotatably receive and support said cylindrical end journals of saidwrist pin therein, said connecting rod member having a continuous andsolid upper end with a cylindrical wrist pin bearing therethroughoperatively received on said connecting rod journal so that saidconnecting rod member is operatively mounted on said wrist pin andbetween said pin bosses while said connecting rod is operativelyretained on said wrist pin and in said piston member by physicalinterference between at least one of said journals of said wrist pin andat least one of said other members of said assembly which inhibits theremoval of said pin from said pin bosses of said piston member.
 2. Thepiston, connecting rod and wrist pin assembly of claim 1, wherein saidouter diameter of said connecting rod journal of said wrist pin isgreater than the internal diameters of said axially spaced bearings ofsaid pin bosses of said piston member so that there is physicalinterference therebetween which inhibits the removal of said wrist pinfrom said piston to thereby maintain said wrist pin and said piston andconnecting rod members in assembly.
 3. The piston, connecting rod andwrist pin assembly of claim 1, wherein said outer diameters of said endjournals of said wrist pin are greater than the internal diameter ofsaid cylindrical wrist pin bearing of said upper end of said connectingrod mounted thereon so that the upper end of the connecting rod isoperatively trapped on said wrist pin and said wrist pin is operativelytrapped between said pin bosses of said piston member.
 4. A metallicpiston, connecting rod and wrist pin assembly comprising a generallycylindrical piston having a pair of laterally spaced support bossesfixed thereto, each of said bosses having an annular opening definingcylindrical bearings of equal internal diameters therein axially alignedwith one another, an elongated connecting rod having a unitary connectorend that is disposed between said laterally spaced support bosses andhaving an annular opening extending through said connector end, saidopening in said connector end having a cylindrical bearing surface thatis generally axially aligned with said cylindrical bearings of saidlaterally spaced support bosses, a one-piece generally cylindrical wristpin extending axially and operatively mounted in said cylindricalbearings of said laterally spaced support bosses, said pin havingintegral and cylindrical end journals of the same outer diameters formedon the opposite ends thereof operatively received in said associatedcylindrical bearings of said support bosses and having an intermediatecylindrical journal between said end journals, said cylindrical bearingsurface of said connector end of the connector rod being operativelymounted on said intermediate journal of said wrist pin so that saidconnecting rod is mounted for limited turning movement on said pin, saidintermediate journal having an outer diameter that differs from theouter diameter of said end journals of said pin received in the bearingsof said bosses to effect the entrapment of said pin and said connectingrod between said bosses and the operative linking of said connecting rodwith said piston.
 5. The piston, connecting rod and wrist pin assemblyof claim 4, wherein said outer diameter of said intermediate journal ofsaid pin and the inner diameter of the cylindrical bearing surface ofsaid connector end of said rod are less than said outer diameter thansaid end journals of said pin so that said pin is trapped on said rodand said rod is trapped between said between said bosses.
 6. The piston,connecting rod and wrist pin assembly of claim 4, wherein said bosses ofsaid piston have walls which face one another, said intermediate journalof said pin operatively mounting said connecting rod thereon beingformed with an outer diameter greater than the outer diameter of the endjournals thereof and with shoulders on the opposite ends thereof thatcan engage said interfacing walls of said bosses to trap the pintherebetween and to thereby retain said connecting rod on said pin andin said piston.
 7. The piston, connecting rod and wrist pin assembly ofclaim 4, wherein said bosses have inner faces opposing one another andwherein said pin is retained within said piston and said connector endof said connecting rod is retained in between said bosses by theengagement of said intermediate journal of said pin with said innerfaces of bosses to prevent the axial movement of said pin beyond theouter limit of said bosses.
 8. The assembly as in any one of claims 2-5,wherein said cylindrical bearings in said support bosses have outerends, and wherein said piston, connecting rod and wrist pin form incombination with one another the sole components mechanicallyinterconnecting themselves into said assembly and wherein said pin isrestrained from moving past the outer end of said cylindrical bearingsin said pin support bosses by the mechanic interconnection of saidcomponents.